1. Technical Field
The present invention relates to components used in the calibration or verification of absolute frequency and amplitude measuring test equipment. More particularly the present invention relates to a highly accurate sine wave generation circuit used in calibrating or verifying the accuracy of test equipment such as RF power meters and spectrum analyzers.
2. Related Art
Existing calibration devices that can generate precise sine waves are typically provided internal to the instrument being calibrated, or as an external attachment. The internal RF calibrator in a test device typically has one power level and is accessible using the front panel space on the instrument. It may be undesirable, however, to use the front panel space which is typically in short supply on portable instrumentation. An external bench top calibrator, further, is typically bulky and requires a wall plug in line voltage for operation. The required line voltage may not be available for a field test instrument.
FIG. 1 shows a block diagram of components of a typical calibrator. The system includes a precise oscillator 2, typically operating at 50 MHz. The output of oscillator 2 is provided to a level modulator 4 that provides a stable voltage output from the oscillator 2 as controlled by a feedback signal. The output of the level modulator 4 passes through an amplifier 6, low pass filter 8, and attenuator 22 to a test port 24. The attenuator 22 is shown as a variable attenuator, allowing a user to set the desired attenuation level. The amplifier 6 increases the output of oscillator 2, while low pass filter 8 removes unwanted harmonics. The variable attenuator 22 is typically included in an external bench top calibrator that connects to a test device, allowing a user to select different output levels as desired during calibration. As an alternative to the variable attenuator 22, a fixed attenuator can be used. A fixed attenuator is more typically included on a calibrator that is internal to a test device.
The feedback signal to the level modulator 4 is provided from an amplifier 16. The feedback signal comes to an input of amplifier 16 from the output of the low pass filter 8 through a detector diode 10 and resistor 14. A filter capacitor 12 removes an AC component of the feedback signal. A capacitor 20 enables amplifier 16 to function as an integrator. A second input to the amplifier 16 is provided from a voltage reference 18. The voltage reference 18 has a voltage value set to control the desired input level of attenuator 22.
It would be desirable to provide components for a calibration device that can provide a precise sine wave with two power levels that does not use up front panel space on an instrument being calibrated, is not bulky, and does not require a line voltage attachment.